This invention relates to a hydraulic press particularly for the mold-pressing of fiber-reinforced synthetic materials. The hydraulic press is of the type in which the pressing force is applied to a press plate by at least one hydraulic press cylinder unit at a location where the cylinder unit is connected to the press plate. The press plate is guided laterally and may be braked by countersupport devices. The connection between the hydraulic press cylinder unit and the press plate is formed by a hydrostatic bearing element including an annular body which is partially filled with hydraulic fluid and which is provided with at least one inner annular sealing element. The plunger-like piston rod of the press cylinder unit extends into the annular body such that a relative angular motion between the press plate and the plunger is ensured.
A hydraulic press of the above-outlined type is disclosed, for example, in U.S. Pat. No. 4,076,780. In such plastic molding presses the parallel run of the press plate is ensured by countersupport devices in the zone in which the pressing force becomes effective. The guidance of the press plate during the rapid-motion closing phase must center the press plate only to such an extent that a shock-free immersion of the tool guides into the countersupport devices is ensured. By virtue of the fact that the press plate, after contacting the countersupport devices, is normally engaged under conditions of static redundancy, ways have been sought to avoid such an occurrence. Thus, it is known to guide the press plate during the rapid closing with hydraulically positioned runner rollers on circular columns arranged on either side of the press plate and, after the penetration of the tool guides into the countersupport devices, to remove the forces applied to the runner rollers, that is, to render such a press plate guide ineffective. It is further known to couple the press cylinder unit with the press plate by spherical joints.
The known constructions, however, cannot provide for a desired free self-orientation (self-setting) of the press plate. Even a known spherical joint connection is to be regarded as a rigid coupling during the prevailing surface pressures.
In another known hydraulic press, disclosed in German patent document No. 3,207,242, which is a counterpart of U.S. Pat. No. 4,457,684, a hydrostatic bearing is utilized for applying the press forces from below to a vertically movable press table. The hydrostatic bearing is formed on the top of the piston of the press cylinder unit and thus has no element into which a component of the press cylinder could penetrate. In this arrangement the sealing problems encountered at the parts which transmit pressing forces are very difficult to overcome, if at all possible.
Further, German patent document No. 1,136,574 discloses a device for transferring pressing forces in hydraulic presses into the movable transverse press head. In the center of the hydrostatic bearing, however, an additional bearing formed as a spherical shell is provided which locally delimits the hydrostatic bearing. Since the hydraulic fluid is admitted to the hydrostatic bearing from the hydraulic cylinder through an axially parallel bore provided in the piston, the piston has to be specifically prevented from being pushed out of the bowl-shaped bearing member. For this purpose a tension anchor is secured centrally to the terminus of the piston and the bearing body. It is a significant disadvantage of this arrangement that it is adapted only for a hydraulic press which is provided with a sole location where pressing forces are transferred from the force-generating unit. This is so because, in case of a simultaneous use of several such arrangements which exert a force parallel to the movable transverse head of the press, stresses are generated in case of oblique orientations of the transverse head as a result of distance variations of the axes of the associated components which in the normal state are aligned with one another.